This is a Phase II proposal for the development of a scanning proton microprobe (SPM) using proton-induced x-ray emission (PIXE) for detection and mapping of trace element distributions in biological samples. The instrument is based on a low-energy electrostatic accelerator producing a 1.5 MeV proton beam for microprobe analysis of samples either in vacuum or at atmospheric pressure. The instrument is intended to detect elements with atomic numbers > 10 with ppm sensitivity. The instrument is designed to have spatial resolution of 5um at atmospheric pressure and 1um for vacuum samples. Thermal damage of samples is to be minimized by rapid scanning of the beam.